Stardust may be basis of life on Earth

Friday 15 December 2006 06.15 EST
First published on Friday 15 December 2006 06.15 EST

Comets could have brought the basic ingredients of life to Earth, scientists revealed yesterday.

The first analysis of samples that Nasa's Stardust mission brought back to Earth from a comet earlier this year has revealed that comets contain a richer range of ingredients than previously thought, including the complex molecules needed to kick-start biology.

The findings will force a re-evaluation of the traditional thinking on comet formation. "We think we know what these things are made of and then suddenly we find that, no, we don't," said Monica Grady, an astronomer at the Open University who worked on the Stardust samples.

Nasa launched Stardust to test the standard concept that comets are just dirty balls of snow left over from the early solar system. It was sent to examine the comet Wild 2 in February 1999.

The probe flew through the tail of dust and debris the comet had emitted and, after travelling 2.88bn miles, returned to Earth earlier this year with a payload of thousands of tiny particles from the comet.

The results of the first investigations of the trapped dust were presented yesterday at the American Geophysical Union's autumn meeting in San Francisco and simultaneously published in the journal Science.

To their surprise, scientists found a huge range of minerals in Wild 2. In particular, the samples showed evidence of aluminium- and calcium-rich minerals that could only have formed at very high temperatures, presumably close to the sun.

Donald Brownlee, an astronomer at the University of Washington and the principal investigator for Stardust, said the traditional ideas that comets were made in isolated parts of the outer solar system would need revision. "As the solar system formed 4.6bn years ago, material [must have] moved from the innermost part to the outermost part. I think of it as the solar system partially turning itself inside out," he said.

Wild 2 also seems to have some of the complex organic molecules that could be precursors to life.

"It's a fairly widely held belief that comets may have played a key role in delivering organics to the early Earth and played a role in getting life started," said Scott Sandford of Nasa's Ames research centre, who led one of the research teams.

When the Earth first formed, it would have been a molten body so hot that any organic materials already present on it would have perished. Any complex organic materials made in space would have had to arrive on the young Earth well after the planet had cooled down. "A lot of our findings support this interesting idea, which is that comets played this key role," said Dr Sandford.

"We don't know how life got started on the Earth. But one would presume that the more complex the things you drop on the Earth, the easier it might be for life to get started. We know that comets and asteroids deliver this sort of material."

Of most interest are the types of organic molecules seen in laboratory simulations of the early solar system, in which scientists irradiate ices containing dirt and dust. These produce a lot of organic compounds including amino acids; Wild 2 seems to contain similar molecules.

Dr Sandford said: "The possible presence of this material in the comet is exciting because it suggests that many [more] of these kinds of compounds that are biologically interesting may well be there."